This invention relates to preparation of tubular beta"-alumina ceramic bodies by the extrusion process. More particularly, this invention relates to preparation of tubular beta"-alumina ceramic precursors, also known as green bodies, which after sintering, have properties which are similar or better than those of tubular bodies produced by isostatic pressing, i.e., density greater than 98% of theoretical, resistivity approximately 5 ohm centimeters at 300.degree. C. and a helium leak rate less than 3.times.10.sup.-9 cc/cm.sup.2 /sec. Still more particularly, the invention relates to an extrusion process by which tubing which is straight and smooth (inside and out) and free from impurities and imperfection is produced in a continuous manner.
Techniques have been developed for the preparation of beta"-alumina tubular ceramic bodies in which a ceramic-binder composition is isostatically pressed in a tubular mold, after which binder is burned off, and then the tubes are sintered. However, that isostatic method is not well suited to the preparation of tubes, particularly those having thin wall thicknesses, because of the formidable problems associated with mold filling prior to isostatic pressing and sample removal after pressing.
U.S. Pat. Nos. 4,158,688 and 4,158,689 to Pett et al, which are commonly assigned with this application, disclose preparation of green bodies of unusual dimensional precision using sacrificial binder similar to that of this invention. In those patents, the green bodies formed include compression molded cordierite ribbed sheets and extrusion molded beta"-alumina precursor particulate tubes. Particulate solids such as beta"-alumina ceramic precursor particulate have special properties inhibiting their facile shaping into dense sintered articles. As opposed to cordierite particulates, they are not as readily wetted by the binder ingredients. Furthermore, certain forms of such beta"-alumina precursor particulates inhibit the flow of sacrificial binder composition containing them. However, neither the binder compositions used with the cordierite nor the extrusion compositions used to make the tubes is optimal for use in extrusion molding of the beta"-alumina precursor particulate tubular green bodies of the subject invention. This invention permits improvements in the extrusion molding of sacrificial binder composition densly packed with powdered beta"-alumina precursor particulate into tubular green bodies, particularly those having thin wall thickness suitable for use in a sodium sulfur battery.